The present invention relates generally to quadrature generators and, more particularly, to a quadrature generator which generates a set of signals in perfect phase quadrature with one another.
A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a transmission channel. A communication signal is transmitted by the transmitter upon the transmission channel to be received by the receiver. A radio communication system is a communication system in which the transmission channel comprises a radio frequency channel defined by a range of frequencies of the electromagnetic frequency spectrum. A transmitter operative in a radio communication system must convert the communication signal into a form suitable for transmission upon the radio-frequency channel.
Conversion of the communication signal into the form suitable for transmission upon the radio-frequency channel is effectuated by a process referred to as modulation. In such a process, the communication signal is impressed upon an electromagnetic wave. The electromagnetic wave is commonly referred to as a "carrier signal." The resultant signal, once modulated by the communication signal, is commonly referred to as a modulated carrier signal. The transmitter includes circuitry operative to perform such a modulation process.
Because the modulated carrier signal may be transmitted through free space over large distances, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
The receiver of the radio communication system which receives the modulated carrier signal contains circuitry analogous to, but operative in a manner reverse with that of, the circuitry of the transmitter and is operative to perform a process referred to as demodulation.
Numerous modulated carrier signals may be simultaneously transmitted as long as the signals are transmitted upon differing radio frequency channels of the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands and have regulated transmission of the modulated carrier signals upon various ones of the frequency bands. (Frequency bands are further divided into channels, and such channels form the radio-frequency channels of a radio communication system.)
A two-way radio communication system is a radio communication system, similar to the radio communication system above-described, but which permits both transmission of a modulated carrier signal from a location and reception at such location of a modulated carrier signal. Each location of such a two-radio communication system contains both a transmitter and a receiver. The transmitter and the receiver positioned at a single location typically comprise a unit referred to as a radio transceiver or, more simply, a transceiver.
A two-way, radio communication system which permits alternate transmission and reception of modulated carrier signals is referred to as a simplex system. A two-way radio communication system which permits simultaneous transmission and reception of communication signals is referred to as a duplex system.
A cellular communication system is one type of two-way radio communication system in which communication is permitted with a radio transceiver positioned at any location within a geographic area encompassed by the cellular, communication system.
A cellular, communication system is created by positioning a plurality of fixed-site radio transceivers, referred to as base stations, at spaced-apart locations throughout a geographic area. The base stations are connected to a conventional, wireline telephonic network. Associated with each base station of the plurality of base stations is a portion of the geographic area encompassed by the cellular, communication system. Such portions are referred to as cells. Each of the plurality of cells is defined by one of the base stations of the plurality of base stations, and the plurality of cells together define the coverage area of the cellular, communication system.
A radio transceiver, referred to in a cellular communication system as a cellular radiotelephone or, more simply, a cellular phone, positioned at any location within the coverage area of the cellular communication system, is able to communicate with a user of the conventional, wireline, telephonic network by way of a base station. Modulated carrier signals generated by the radiotelephone are transmitted to a base station, and modulated carrier signals generated by the base station are transmitted to the radiotelephone, thereby to effectuate two-way communication therebetween. (A signal received by a base station is then transmitted to a desired location of a conventional, wireline network by conventional telephony techniques. And, signals generated at a location of the wireline network are transmitted to a base station by conventional telephony techniques, thereafter to be transmitted to the radiotelephone by the base station.)
Increased usage of cellular, communication systems has resulted, in some instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. By more efficiently utilizing the frequency band allocated for radiotelephone communication, the transmission capacity of an existing, cellular communication system may be increased.
The transmission capacity of the cellular, communication system may be increased by minimizing the modulation spectrum of the modulated signal transmitted by a transmitter to permit thereby a greater number of modulated signals to be transmitted simultaneously. Additionally, by minimizing the amount of time required to transmit a modulated signal, a greater number of modulated signals may be sequentially transmitted.
By converting a communication signal into discrete form prior to transmission thereof, the resultant, modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of a communication signal that has not been converted into discrete form. Additionally, when the communication signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
Converting the communication signal into discrete form is typically effectuated by an encoding technique, and apparatus which effectuates such conversion is typically referred to as an encoder. An encoded signal generated as a result of an encoding technique may, for example, be in the form of a discrete binary data stream. The elements (i.e., bits) of the discrete binary data stream represent various characteristics of the information signal.
Quadrature amplitude modulation (QAM) is a modulation technique which may be advantageously utilized to transmit efficiently a communication signal encoded into discrete form. More particularly, one particular QAM modulation technique is a .pi./4-DQPSK (for differential quadrature phase shift keying) modulation technique. Such modulation technique has been selected as a modulation standard for several cellular, communication systems. In a .pi./4-DQPSK modulation technique, the binary data stream into which the communication signal is encoded is separated into bit pairs. Such bit pairs are utilized to cause phase shifts of a carrier wave in increments of plus or minus .pi./4 radians or plus or minus 3.pi./4 radians according to the values of individual bit pairs of the encoded signal.
Such phase shifts are effectuated by applying the binary data stream comprised of the bit pairs to a pair of mixer circuits. A sine component of a carrier signal is applied to an input of a first of the mixer circuits, and a cosine component of a carrier signal is applied to an input of a second mixer circuit of the pair of mixer circuits. (The sine and cosine components of the carrier signal are in a relative phase relationship of ninety degrees with one another.)
A quadrature generator is utilized to apply the sine and cosine components of the carrier signal to the first and second mixer circuits of the pair of mixer circuits, respectively.
A quadrature generator may be formed of a resistor-capacitor pair in which the value of at least either the resistor or the capacitor is variable as a function of voltage. The frequencies of oscillation of the signals generated by a quadrature generator are dependent upon the values of the resistor-capacitor pair, and, as the values of the resistor and capacitor of the resistor-capacitor pair are functions of voltage, the range of frequencies of the signals generated by the quadrature generator is dependent upon voltage levels of frequency-controlling voltages applied to the quadrature generator.
As the circuitry of apparatus, such as a radiotelephone utilized in a cellular, communication system, of which the quadrature generators form a portion, are constructed to be operated at ever-lower voltage levels, the range of values of which the resistor or capacitor of the resistor-capacitor pair can take is increasingly limited. The range of frequencies of signals generated by a quadrature generator so constructed is increasingly limited.
A quadrature generator may alternately be constructed of a flip-flop pair arranged such that the outputs of each flip-flop of the flip-flop pair are applied to inputs of the other flip-flop of the flip-flop pair. A clock signal is also applied to each of the flip-flops of the flip-flop pair wherein the clock signal is inverted prior to application to one of the flip-flops. Outputs of the respective flip-flops of the flip-flop pair are in a ninety degree phase relationship (and, hence, are in phase quadrature) when the duty cycle of the clock signal applied to the flip-flops is of a fifty-fifty duty cycle. That is, the clock signal must be of a high logic level for exactly half of the period of the clock signal and be of a low logic level for exactly half of the period of the clock signal.
Any variation in the duty cycle of the clock signal causes the signal output by the respective ones of the flip-flop pair to be out of phase quadrature (i.e., in a phase relationship other than a ninety degree phase relationship) with one another. When the duty cycle of the clock signal is significantly different than a fifty-fifty duty cycle, the signals generated by the flip-flop pair are significantly out of phase quadrature.
Clock oscillators which generate clock signals will not in general produce clock signals exactly of the fifty-fifty duty cycle. Additionally, the duty cycle of the clock signal generated by a clock oscillator may vary as the clock oscillator ages or as a result of circuit placement of the clock oscillator.
What is needed, therefore, is circuitry operative to generate reliably a pair of signals in perfect phase quadrature with one another.